Fresh water shortage affects roughly one-third of the world's population and is becoming more critical in recent years due to drought, population increase, development of population centers in arid areas, and pollution. About 97.5% of the water on earth is saltwater and the remaining about 2.5% is fresh water. Therefore, a practical, economically viable desalination process is crucial to overcoming this crisis. For these reasons, and others, desalination is an important issue.
Desalination can be accomplished by a number of methods known in the art. Conventionally, the most commonly used desalination process involves variations of one or more thermal processes, for example evaporation or distillation. These thermal methods involve a high energy requirement and, therefore, the cost of desalination historically has been high, limiting desalination as a viable large scale option in many parts of the world. Other known desalination methods include membrane processes, e.g. filtration and electrical separation methods such as reverse osmosis (RO). Although these processes gradually have improved in energy consumption, large scale operations still incur high operating costs and experience obstacles such as membrane fouling, capacity limitations, and expensive construction materials.
Freeze crystallization is a thermal desalination process wherein saline water is chilled to a temperature sufficient for the saline water to freeze. It is based on the fundamental principle that the structure of an individual ice crystal does not accommodate salts. As a result, ice crystals, formed after water in brine freezes and is separated from the resulting salt crystals, consist of pure water. These pure ice crystals can then be separated from the salt crystals and melted to form pure water.
In operation, freeze crystallization can be achieved by direct cooling or indirect cooling. Once the saline water is frozen and ice crystals of non-saline, purified water have formed, the ice crystals are separated via known means in the art from the resulting residual brine or solid salt crystals. The separated ice crystals are then melted to produce non-saline, drinking or potable water (for example, water with less than 100 ppm salinity).
Despite its potential, the freeze crystallization process has not been successfully implemented on a large, commercial scale. Historically, there have been three principal challenges (1) the difficulty in using refrigeration systems to efficiently freeze large quantities of saline water without forming large chunks of ice; (2) high operating costs; and (3) equipment/plant complexity. Systems that utilize indirect cooling (e.g external refrigeration) require large vessels to hold the saline water, which results in inefficient heat transfer between the refrigerant and the saline water. Direct refrigerant injection into the saline water exhibits a higher heat transfer surface area, but requires the additional step of the recovering refrigerant from the saline water. In both methods, the complexity of handling and separating the brine/ice slurry remains an obstacle.
Because of the drawbacks of the existing desalination and freeze crystallization methods discussed above, there is a need for a highly efficient and cost effective desalination method that allows fresh drinking (i.e. potable) water to be produced from saline water utilizing a turbo freeze process. The method disclosed herein solves the obstacles of conventional freeze desalination technologies by providing an energy efficient method to form ice crystals without limitations of existing systems. Furthermore, close to 100% of the water can be frozen since heat transfer in the process does not require the presence of the liquid phase. The resulting dry mixture of ice and salt crystals can be mechanically separated, thus reducing the equipment complexity often required for conventional freeze crystallization processes.
It therefore is an object of the present disclosure to provide a novel, cost-effective and efficient process for desalinating saline water using a turbo freeze, vapor-compression-expansion method, wherein direct contact between a cold expanding stream of compressed fluid and saline water droplets allows for the simultaneous production of ice crystals (containing pure water) and salt crystals, which can then be readily separated. As described herein, the energy required for the saline water cooling and water freezing is provided by the expanding and cooling of compressed fluid and vaporization of the fluid. The fluid is a refrigerant that will condense under conditions similar to those used in conventional air conditioner units. More specifically, when the fluid refrigerant is compressed and at least partially condensed, it will create a chilled temperature (i.e. “fast-cooling”, “hyper-cooling” or “turbo-cooling”) upon expansion. The partially condensed refrigerant is directly mixed with the saline water droplets at a sufficient slip velocity such that the chilled refrigerant reduces the temperature of the saline water to produce frozen ice particles and salt crystals within a reduced residence time as compared to other methods.
The process disclosed herein is fast, reduces energy requirements, has less complex and smaller equipment, a high fresh water production (potential of 100%), and is more cost efficient for treatment of high salinity water than other methods.